Glass Fiber Reinforced Polymer Laminates Research Articles

Experimental study of drilling behaviors and damage issues for woven GFRP composites using special drills

Glass fiber reinforced polymer (GFRP) composites are becoming more attractive in modern engineering fields due to their outstanding mechanical/physical properties. However, a thorough understanding of their drilling machinability is seriously lacking in the research community. The present work aims to address the drilling behavior of woven GFRP composites under varying cutting speeds and feed rates. Machining studies were conducted using two different diamond-coated special tools involving a double point angle drill and a dagger drill. The drilling machinability of GFRPs was comprehensively analyzed in terms of cutting forces, machining temperatures, drilling-induced damages, dimensional accuracy, and hole wall morphologies. Theoretical analyses and experimental characterizations of delamination damage were conducted for both special drills. A special attempt was made to clarify the impact of different drill shapes on the damage formation and extension of cut GFRP materials. The results obtained can supplement the expertise of composites machining and guide the damage-free drilling of GFRP laminates for academia and industry.

Characterization of Eggshell Composition at Different Size and Concentration in Fire-Retardant Application

This paper explores the thermal characterization of eggshell composition at different sizes and concentrations in glass-fiber-reinforced polymer (GFRP) composites as a potential flame retardants material. The eggshell powder (ESP) with different sizes (53 μm and 100 μm) and concentrations (10 wt% and 30 wt% ) was mixed with epoxy and laminated with four layers of 800 gsm E-glass. Thermo-gravimetric analysis (TGA) has been performed to analyze the thermal properties of the GFRP laminates. The finding reveals that incorporating the ESP in the GFRP composites material does not affect the epoxy matrix’s thermal properties as all the trails deteriorate between 450 °C to 480 °C. The combination of 100 μm filler size and 30 wt% ESP improved the fire resistance by 4.68% when compared to 53 μm size and 30 wt% ESP. In addition, the increment of filler loading from 10 wt% to 30 wt% of the 100 μm filler size showed an improvement of 10.9%. This concludes the potential of ESP as a fire retardant material.

Investigation on the structural failure behaviour of pultruded circular tubular GFRP multiplanar truss bridges with non-metallic connections through finite element modelling

The finite element (FE) method was used to investigate the failure of member connections of five glass fibre-reinforced polymer (GFRP) multiplanar truss bridges (MTB) with non-metallic connections. Three-dimensional FE models for the MTBs were developed and validated against previous experimental results and were accompanied by multiple parametric studies. Adhesive failure of the brace occurs progressively around the perimeter of the brace and GFRP laminate wrapping failure was not observed before adhesive bond failure of the brace. Furthermore, the failure sequence of the GFRP laminate layer and the adhesive bonding between the brace and chord was found to be dependent on the brace wall thickness. Extent of the predicted fibre tensile rupture of the GFRP laminate was greater for cement mortar grouted GFRP rods than its non-GFRP grouted counterpart.

Structural Behaviour of Polystyrene Foam Lightweight Concrete Beams Strengthened with FRP Laminates

Lightweight concrete (LWC) is one of the most important building materials nowadays. Many research studies were focused on LWC produced using lightweight aggregates. However, limited work was cited for LWC produced using polystyrene beads. In this study, LWC beams strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) were experimentally tested to investigate the improvement in their flexural and shear behaviours. LWC in this investigation was achieved by partial replacement of normal aggregate by polystyrene beads and resulted in approximately 30% less weight compared to Normal weight concrete. Fourteen Reinforced Concrete (RC) LWC beams of 100 mm by 300 mm cross section having an overall length of 3250 mm were tested under four-point bending. These beams were designed, detailed, and tested to obtain flexural and shear mode of failure. These beams were divided into two groups based on the intended failure mode. In each group, six beams were strengthened using CFRP and GFRP laminates, while the remaining one beam was used as control. The tested parameters were the type of FRP, the width of the laminates used in shear strengthening, and the number of layers used in flexural strengthening. It was found that strengthening of LWC beams using CFRP and GFRP layers resulted in increasing the loading capacity and decreasing deflection as compared to control. The strengthening with CFRP and GFRP is also suitable in reducing the crack width and crack propagation which is more significant in LWC beams. The experimental results were also compared with the expressions in codes for forecasting the strength of LWC beams and it was that these expressions are compatible with the experimental results.

Mechanical behavior of Glass fiber‐reinforced polymer‐timber‐steel tube‐concrete composite columns under axial compression

AbstractThis paper proposes a novel glass fiber‐reinforced polymer (GFRP)‐timber‐steel tube‐concrete composite column (GTSC) that combines an outer steel tube with GFRP fabric reinforcement, an inner timber core, and concrete for the rest of the space. The mechanical behavior of eight specimens of novel composite columns was investigated considering diameters of timber cores, layer numbers, and angles of GFRP laminates. The results showed that as the size of the timber core increased from 0 to 50 mm, the ductility and the energy dissipation capacity of the composite column maximum increased by 61.5% and 63.4%, respectively. Conversely, the ultimate load carrying capacity and stiffness decreased. It was also found that the timber core weakened the strength of the confined concrete. However, timber core reduced the weight of columns, which slowed down the reduction of the ultimate load carrying capacity of per unit GTSCs, with the stiffness remains unchanged. GFRP laminates with an angle of 60° provide the best stiffness and ductility, and the layer numbers of GFRP has a positive effect on GTSC's mechanical performance. In addition, an analytical model was proposed based on the ultimate equilibrium method, which accounts for the ultimate bearing capacity of GTSC based on the influence of timber. Analyses revealed a good correlation between analytical and experimental results.

Influence of elevated temperatures on the mechanical properties of glass fibre reinforced polymer laminates produced by vacuum infusion

One of the main concerns about the use of fibre reinforced polymer (FRP) materials in civil engineering is their behaviour when subjected to elevated temperatures and under fire exposure. In fact, considerable reductions in the mechanical properties of FRP materials have been reported even at moderately elevated temperatures (e.g. 50–150 °C), due to the glass transition of their polymeric matrices. Most previous studies on this topic have focused on the mechanical characterization at elevated temperatures of quasi-unidirectional pultruded FRP composites, namely profiles, rebars and strips; much less data is available about FRP materials produced with different methods with more balanced fibre architectures. This paper aims at contributing to fulfil this knowledge gap by presenting experimental and analytical investigations about the effects of elevated temperatures on the mechanical properties of glass-FRP (GFRP) laminates produced by vacuum infusion, with a balanced fibre architecture, representative of typical GFRP face sheets of sandwich panels used in civil engineering structures. The experimental programme included (i) tensile tests up to 300 °C, (ii) compressive tests up to 250 °C, and (iii) shear tests up to 200 °C, all under steady state conditions, as well as dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The results obtained confirm that the mechanical properties of the GFRP laminates are severely affected by the temperature increase, especially those that are matrix-dependent: compared to room temperature, at 200 °C, the shear modulus was reduced by 88%, whereas the compressive strength and modulus were reduced by 96% and 67%, respectively. The tensile properties (fibre-dominated) were much less affected - at 200 °C, the tensile strength and modulus were reduced by 40% and 48%, respectively. Finally, the suitability of four empirical models and of a design-oriented temperature conversion factor to take into account the variation of the GFRP mechanical properties with temperature was assessed. Overall, the empirical models presented good agreement with the test data, and the temperature conversion factor provided conservative estimates of the material properties, attesting its suitability for design purposes.

Hygrothermal and temperature effects on load-carrying capacity and failure mechanism of open-hole laminates prepared with narrow tape prepreg of GFRP

The tensile and compressive properties of the open-hole laminates fabricated with glass fiber reinforced polymer (GFRP) under various temperature/humidity coupled conditions are important design parameters for GFRP structures in helicopter. In current work, a comprehensive experimental study about the effects of temperature and hygrothermal on the open-hole mechanical property of HS6/AC318 glass/epoxy multidirectional laminates was conducted. The load-displacement responses, fracture morphologies and failure mechanisms of quasi-isotropic GFRP laminates with a center hole under tensile and compressive loads at the five environmental conditions were compared, and the effects of the temperature and hygrothermal on the open-hole tensile (OHL) strengths and open-hole compressive (OHC) strengths of GFRP laminates were investigated. In addition, the relevancies between the OHL and OHC strength of open-hole laminates and the basic material properties of unidirectional laminates were explored, and some links between the OHL strength and interlaminar fracture toughness were found. The results showed that the OHC strength of open-hole laminates was directly proportional to the 0° compressive strength of unidirectional laminates. Based on the experimental data and analysis an empirical formula of the OHC strength at arbitrary temperature and humidity combined conditions was proposed.

Propagation of tunnelling cracks in composite materials under strain and force-controlled cyclic loading

In this paper, the propagation of tunnelling cracks in composite laminates subjected to uniaxial force-controlled and strain-controlled cyclic loading is analysed considering both interacting and non-interacting cracking scenarios. Glass fibre reinforced polymer laminates with [Formula: see text] layup are used in the analysis. For non-interacting cracks, a strain-controlled test under a single strain value is found to be sufficient to produce the slope of the Paris-Erdogan law type curve relating the crack front growth rate (CGR) with the energy release rate (ERR), but within a limited range of ERR. Whereas, a force-controlled test is found more suitable when characterising the variation of the CGR at a certain ERR value. For closely spaced cracks, commonly seen crack interaction scenarios are identified and the CGR of the crack front is investigated for both loading control modes. It is found that the CGR decreases when volume averaged stresses are reduced ahead of the crack front due to shielding effect at high crack densities. A relationship between these two parameters is presented. It is further found that the CGR of two collinear crack fronts growing towards each other for coalescence do not influence each other. But when the two crack fronts are not collinear the CGR of the crack fronts is significantly affected after they have grown past each other.

Translaminar mode-I fracture toughness experiment of pultruded GFRP laminates using extended compact tension specimen

This paper conducted experiments and theoretic analysis of the fracture performance of pultruded GFRP (Glass Fiber Reinforced Polymers) laminates. The mode-I fracture toughness of pultruded GFRP laminates was evaluated by the eccentrically loaded, Extended Compact Tension (ECT) specimen. The geometry of specimen and experimental procedure refer to ASTM E1922. A total of 16 specimens were tested, which included 0° and 90° roving orientation. Finite element analysis with the Virtual Crack Closure Technique (VCCT) was implemented to calibrate the experimental results. A correction function for normalized notch length was proposed for pultruded GFRP translaminar fracture toughness. Carpet plot was designed to discuss the relationship between fiber volume ratio and fracture toughness, which demonstrated that the fracture toughness strongly correlated with the fiber volume ratio. Also, the effect of fraction of 0° layers on fracture toughness and material orthotropy was studied. The relationship of material orthotropy parameter and fracture toughness was discussed.

Effect of simulated hygrothermal environment on the flexural and interlaminar shear strength of particulate-filled epoxy-coated GFRP composites

The application of glass fiber-reinforced polymer (GFRP) composites in civil engineering and construction has gained significant interest in the last decade. Such applications require information on the long-term performance and durability of fiber composite materials because these materials are directly exposed to aggressive environments. This comparative study evaluated the effect of a simulated hygrothermal environment on the durability of GFRP composite laminates with and without a particulate-filled epoxy coating. The three-point bending test was conducted to characterize the degradation of interlaminar shear strength and flexural properties of GFRP composites before and after conditioning at combined high moisture and temperature for 0, 1000, 2000, or 3000 h. Dynamic mechanical analysis was also conducted to characterize the changes in the thermomechanical properties of the composite laminates. The results show that hygrothermal conditioning had a negative impact on the flexural and interlaminar shear strength of uncoated GFRP laminates, but this could be efficiently minimized with the use of a particulate-filled polymer-epoxy coating.

Mechanical Behavior of GFRP Laminates Exposed to Thermal and Moist Environmental Conditions: Experimental and Model Assessment.

This paper presents an experimental and analytical study about the mechanical response at a different temperature on glass fiber-reinforced polymer laminates. The effect of different environmental conditions on compressive, tensile, stiffness, and viscoelastic behavior (storage modulus, loss modulus and damping ratio) of laminates were investigated. Before testing, laminates were preserved in a deep freezer at −80 °C, −20 °C, 0 °C, and room temperature (25 °C) for up to 60 days. Results confirmed that temperatures ranging from −80 to 50 °C, which were below the glass transition temperature of the epoxy resin, did not significantly affect the compressive, tensile, and stiffness performance of all laminates. When the testing temperature increased to 100 °C, the properties were decreased significantly due to the damaging of the fiber/matrix interface. Additionally, results obtained from dynamic mechanical analyses tests showed a drop-in storage modulus, high peaks in loss modulus and high damping factor at the glass transition region of the epoxy resin. The highest storage modulus, two phases of glassy states and highest damping ratio on the −80/G group of laminates were obtained. The accuracy of experimental results was assessed with empirical models on the storage modulus behavior of laminates. The empirical model developed by Gibson et al. provided accurate estimates of the storage modulus as a function of temperature and frequency. The remaining empirical models were less accurate and non-conservative estimations of laminates stiffness.

A model of strength degradation for glass fiber reinforced polymer composite laminates under fatigue loading

In order to overcome the shortcoming of traditional strength modeling of glass fiber reinforced polymer (GFRP) composite laminates that require a large amount of strength test data, a strength degradation model on the account of residual stiffness degradation data is proposed. First, fatigue damage growth can be described by the gradual reduction of the stiffness and strength, and damage expressed by the two degradation methods are equivalent. Second, according this assumption, the connection between the two damage indices is established, and then a four parameters strength degradation model is obtained. Finally, the proposed model is validated by the applying experimental data of GFRP laminates, and the precision of proposed model is contrasted with other four models. Verification results indicate that if the residual stiffness degradation data is known, the residual strength degradation law can be predicted by a small number of residual strength tests and the presented model has better applicability and higher fitting accuracy.

Lightweight, Fiber-Damage-Resistant, and Healable Bio-Inspired Glass-Fiber Reinforced Polymer Laminate.

Glass-Fiber-Reinforced Polymer (GFRP) laminates are widely used in the automotive and marine industries such as auto bodies and boat hulls. Decreasing the weight and improving the reparability of GFRP parts will cut down material usage, fuel consumption and repair costs. This study shows a bio-inspired helicoidal stacking configuration that significantly improves the impact performance and fiber damage resistance of GFRP laminates. For similar impact performance in terms of perforation energy, the helicoidal GFRP laminate is 20% lighter than the conventional quasi-isotropic GFRP laminate. Upon impact, delaminations and matrix splits link-up and grow extensively throughout the helicoidal laminate. This effectively reduces fiber damage and improves impact performance. Because helicoidal GFRP laminates are resistant to fiber damage and composite healing agents can effectively repair non-fiber damage, embedding healing agents into helicoidal GFRP results in lightweight, inexpensive and healable laminates.

Observations of fatigue damage development in woven glass fiber-reinforced polymer composite using transmission light photography technique

Fatigue behaviour is an important aspect for consideration while handling glass fiber-reinforced polymer (GFRP) composites. A combination of (0°/90°) and (±45°) plies are gaining practical interest in engineering applications due to their excellent mechanical properties. Properties of individual above-stated plies primarily influence the behaviour of laminates. Fatigue behaviour of on-axis (0°/90°) and off-axis (±45°) fiber orientated woven GFRP laminates was investigated. Tensile and tension-tension fatigue tests were performed on the prepared test coupons according to the ASTM standard. Fatigue test was conducted until failure at maximum applied stress of 90%, 70%, and 50% of the respective ultimate strength. Damage growth was monitored in the term of debond zones by employing the transmission light photography technique. Testing process was recorded by using a digital camera. The results were established in terms of the stress-strain curve, Wöhler curve, debond zones and dynamic stiffness. The fatigue strength of off-axis laminates was much lower than the on-axis laminates. Global damage has occurred in (0°/90°) laminates while (±45°) laminate witnessed localized damage. At higher stress levels, samples were failed at shorter lifespan with aggressive damage; however, on-axis specimens survived with longer fatigue life.

Research on failure modes and ultimate strength behavior of typical sandwich composite joints for ship structures

This paper presents an experimental and numerical investigation on failure behavior and ultimate strength of fiber-reinforced sandwich composite structures. Tests were conducted on the sandwich L-joints with Glass Fiber-Reinforced Polymer (GFRP) face sheet and Polyvinyl chloride (PVC) foam core. In the numerical simulation, a progressive damage method is established to simulate the strength and failure behavior of sandwich L-joint, different failure criteria are used to deal with the various failure modes of GFRP laminates. Cohesive zone model and viscoelastic material model are combined to model the face-core adhesive interface, hence the influence of fracture and debonding of adhesive to the total strength of sandwich joints is considered by the numerical modelling. On the base of these theories, the finite element model of sandwich L-joint has been successfully established, and the numerical predictions present a good agreement with the test results on both structural strength and failure process.

Numerical investigation of low-velocity impact in symmetric and asymmetric GFRP laminate with and without pre-crack

Damage induced in symmetrical and asymmetrical glass fiber reinforced polymer (GFRP) laminate overlow-velocity impact (LVI) using a mild steel impactor is investigated. Numerical simulation is done using 3-Dfinite element analysis software LS-DYNA. Orientations for symmetrical and asymmetrical laminate were [(0,90)/(+45,-45)/(+45,-45)/(0,90)//(90,0)/(+45,-45)/(+45,-45)/(90,0)] and [(0,90)/(+45,-45)/(+45,-45)/(0,90)//(+45,-45)/(90,0)/ (90,0)/(+45,-45)] respectively. Two samples each from symmetric and asymmetric laminate with and without pre-crack were numerically simulated. Induced circular pre-crack was modeled in the midplane of the laminate. A circular specimen of radius 75mm with clamped area of 376mm2 is modeled. The velocity of 5 m/sec was assigned to the hemispherical headed cylindrical impactor. The result shows that symmetrical laminates absorb high energy and damage area for it is 24.06 % less than asymmetrical laminate for laminate with pre-crack in mid plate. This study concluded symmetrical laminate design is better for structural purpose as compared to asymmetrical design. Results of proposed investigation are directly applicable in aircraft, automobiles and space equipment. Copyright © 2017 VBRI Press.

Shearography non-destructive testing of thick GFRP laminates: Numerical and experimental study on defect detection with thermal loading

Thick composite materials are commonly used as load-bearing structures in marine applications. Developing a suitable and sophisticated non-destructive testing (NDT) method for thick composites is an urgent challenge to improve the safety, reliability and maintenance of these structures. Digital shearography has become an important NDT technique for detecting defects in thin composite materials because of the advantages of high sensitivity to deformation change, and whole-field measurement. So far, the efficacy of shearography for thick composite inspection (e.g. thickness as more than50 mm) has not been fully characterised. This paper combines finite element methods (FEM) and experimental tests to investigate the defect detection capabilities of shearography for inspecting thick glass fiber-reinforced polymer laminates. A thermal–mechanical model was established by computing equivalent thermal and mechanical properties and was evaluated by experimental shearography testing. In order to reliably simulate major defects in thick composite, flat bottom holes were manufactured in the specimen. Both simulations and experiments show that shearography is a promising technique to inspect thick composites. The thresholds for defect-induced phase change and the corresponding defect-induced deformation are determined for shearography testing of thick composites in this paper. Afterwards, the effect of mechanical boundary conditions on defect-induced deformation is studied by FEM.

Influence of milling process parameters and significance of tools to improve the surface quality of GFRP composites

The anisotropic nature of polymer composites presents many challenges for manufacturers to adopt appropriate machining processes. In the present investigation, end milling experiments were conducted on glass fiber reinforced polymer laminates with five varieties of customized cutting tools with different angles of rake and clearance. The performance of the tools was evaluated in terms of their machining force, surface roughness and delamination factor at spindle speeds in the range of 690–2500 rpm. From the observations, relatively high rake and angled clearance tools performed better than the rest of the tools under consideration in terms of delamination and machined surface finishing. The milling operations performed at a spindle speed of 1950 rpm produced better surface quality. Observations from SEM graphs, exposed surface defects due to milling, generated at lower spindle speeds of 690 rpm and at higher spindle speeds of 2500 rpm with the tool signature of low angle rake and angled clearance tools out of all five tools considered for the experiments.

Improved performance of confinement of RC beam with glass fibre reinforced polymer laminates

In recent times, the need and usage of modern composite materials, including the FRP [Fiber Reinforced Polymer], have gained more attention in the construction field for strengthening the RC beam. From previous research, a conclusion was drawn stating that the exterior bond of FRP would increase the element efficiency of the RC beam. But the drawback for Fiber Reinforced Polymer RC beam is due to its stress–strain nature, and lack of yield plateau affects the ductility of the RC beam. The Fiber Reinforced Polymer laminates are made of various materials like glass fiber, carbon, aramid fibers, which helps in altering the linear behaviour of the material to the non-linear behaviour of the material. The present research work tries to analyze the Reinforced concrete RC beam by Glass Fiber Reinforced Polymer [GFRP] laminates. The experimentation of the research is done by studying the performance and behaviour of RC beam strengthening with GFRP laminates. The experimentation is done through 12 T-beams with 130 mm x240mmx3200mm in dimension. The testing of the beam is done through cyclic loading to obtain the flexural behaviour of the beam. Where the direction of the fiber, various ratios of reinforcement, GFRP laminates were added to the beam in order to determine the strengthening process of the beam. The outcome of the experimental setup shows that the GFRP laminates strengthen the beam and also enhances the flexural response of the samples when compared to the other control samples. Also, the load, cracking, energy absorption capacity, stiffness, and ductility of the RC beam increases this is due to the influence of bond length. The bond length has a greater impact on the RC beam properties when compared to the other strengthening reinforcement property.

Influence of Loading Direction on Impact Strength and Small Span Length Variation on Flexural Strength in GFRP Laminate

Abstract This experimental work focuses on the two aspects of glass fiber–reinforced polymer (GFRP) laminate. The first is the effect of GFRP plate thickness and impact loading direction on impact strength, and the second is the effect of GFRP panel thickness and small span length variation on flexure strength. The GFRP laminate thicknesses considered for experimentation were 2, 3, and 4 mm. Both Izod and Charpy impact tests were conducted by considering edgewise (lateral impact) and flatwise (normal impact) impact loading directions. The quasistatic flexural tests were conducted using a three-point bend method for two small span length variations, viz., 50.8 and 56 mm. Impact strength of 2-mm-thick GFRP laminate under the edgewise loading condition showed 13 and 134.1 % more for Charpy and Izod, respectively, compared with 4-mm GFRP samples. In contrast, under the flatwise loading condition, 2-mm-thick specimens showed 14 and 54 % more impact strength when compared with 4-mm-thick GFRP specimens. GFRP samples with 2-mm thickness showed 59.85 and 59.6 % enhancement in flexural strength for span lengths of 56 and 50.8 mm, respectively, when compared with 4-mm-thick GFRP panels. Improvement in flexural strength between span lengths 50.8 and 56 mm for thicknesses 4 and 2 mm is 26 and 42 MPa, respectively, which is a 10 % improvement. Analysis of variance statistical analysis showed that the loading conditions and thickness of GFRP panels in the case of impact and small span length variation, and thicknesses in the case of flexural strength, is the significantly influencing parameters.